Aerodynamic drag force acting on cylinders (including infinitely long ones) is usually separated to an axial component acting along the cylinder axis and a normal component acting in the radial direction. Generally speaking the radial component affects fiber drift rate caused by wind, and fiber fall rate caused by fiber weight, while the axial component affects tension build up along the fiber. A large amount of data exist concerning the normal component and the values given by different sources usually match. This is not the case concerning the axial component, where there are only few sources, most of which are based on measurements made on cables towed in water, and differ much from each other, and thus these cannot be used for practical purposes.
It is one of the difficulties in carrying out accurate measurements of axial drag acting on inclined elongated bodies that, in most cases, the radial drag component is higher by an order of magnitude than the axial one and tends to cause parasitic loads on measuring equipment, which result in erratic axial drag reading.
Also, formulae derived from measurements made on cables and strings and not the filament of interest, do not take into account the nature of the specific filament surface, especially when it has been deployed from an air borne dispenser and contains remainders of the adhesive applied to it during the winding process.
There is provided a method for measuring the aerodynamic drag component parallel to the axis of filament and means for carrying out such measurements.
For carrying out the measurement, there is used a conventional sub-sonic wind tunnel, in conjunction with auxiliary devices, such as specially constructed pulleys, which are positioned either in the wind tunnel or on its outside. The filament is suspended while being supported by said pulleys, the direction of the wind being at a certain angle respective the axis of the filament.
According to one embodiment of the invention, the filament is suspended and tensioned between two such pulleys, and passes through openings in the walls of the wind tunnel, so that the filament makes an angle with the axis of tunnel.
The pulleys used are of special construction, so as to minimize frictional forces. Furthermore, the construction is such that when positioned in the wind tunnel, such pulley unit causes a minimum disturbance on the flow of air in the tunnel.
As set out above, there can be used the following arrangements:
In this case the up-stream pulley unit may cause some disturbances of wind flow in the tunnel section used for the measurement. The filament, such as an optical fiber, is tensioned by means of two weights, underneath each pulley. The weight at one end being slightly heavier than the other causing it to rest on a precise scale. In this position the scale is zeroed. At this stage the wind tunnel fan is turned on to cause an air stream of known velocity along the tunnel test chamber. When desired air velocity is reached the scale reading is registered and the filament horizontal deflection is registered using a TV camera, calibrated before against a mm scale. Then drag components can be estimated using the following expressions: EQU D=W/L
where:
where: